Interaction Between the Supraspinatus and Infraspinatus Tendons Effect of Anterior Supraspinatus Tendon Full-Thickness Tears on Infraspinatus Tendon Strain

Background: Rotator cuff tears are common and not well-understood shoulder problems. Structural and mechanical inhomogeneity of the supraspinatus tendon complicates accurate prediction of risk of tear propagation and may affect appropriate clinical treatment. Hypothesis: We propose that interactions between the supraspinatus and infraspinatus tendons are critical to load bearing at the glenohumeral joint and warrant investigation. Study Design: Controlled laboratory study. Methods: Principal strains in the infraspinatus tendon of cadaveric human shoulders were evaluated with increasing anterior full-thickness supraspinatus tendon tear sizes and loading to evaluate whether a mechanical interaction between the supraspinatus and infraspinatus tendons exists. A constant nominal load was attached to the infraspinatus throughout all experimental conditions. Results: Increasing supraspinatus tendon tear width from intact and 33% to 66% and increasing supraspinatus tendon load caused an increase in maximum and a decrease in minimum principal strain in the infraspinatus tendon. Increasing the supraspinatus tendon tear size from intact to 33% of the width had no significant effect on infraspinatus tendon strain. Conclusion: Supraspinatus and infraspinatus tendons mechanically interact. The observed significant increase in maximum and decrease in minimum principal strain was concurrent in both the infraspinatus and supraspinatus tendons. Clinical Relevance: Changes in infraspinatus tendon strain associated with increases in supraspinatus tendon tear size and loading may shield a torn supraspinatus tendon from further injury or may have an effect on the load-bearing capacity of the infraspinatus tendon. Results also imply that the effect of a small tear that does not compromise the confluent region between the supraspinatus and infraspinatus tendon may be localized, eliminating its effect on infraspinatus tendon strain.